1. Field of the Invention
The field of the invention is digital controllers of the type for controlling the motion of a power tool and a workpiece support along selected axes of motion.
2. Description of the Prior Art
Positioning control has been associated primarily with numerical controls for metal-cutting machine tools. State of-the-art computer-based numerical controls (CNCs) are shown and described in Dummermuth et al, U.S. Pat. No. 4,038,533, issued July 26, 1977, and Bernhard et al, U.S. Pat. No. 4,228,495, issued Oct. 14, 1980. These controls provide full contouring control for a cutting tool as it moves along a complex path determined by a "part program." Numerical controls of the prior art can also be used for point-to-point positioning--drilling operations are one example of this type of positioning control.
Earlier numerical controls consisted primarily of a general purpose digital computer with collections of electronic hardware that interfaced the computer to the servomechanisms on the controlled system. Performance has been improved and real costs reduced for numerical controls by using large scale integrated (LSI) processor components. These LSI processors, however, require their own machine language programs. As programming development costs increase and hardware technology advances, one approach to improving positioning controls is to employ improved hardware circuits in the axis modules interfacing a central numerical control processor to the servomechanisms governing motion along the respective axes.
With the microelectronics revolution of the late 1960's and early 1970's, a new type of digital controller, known as the programmable controller (PC), was developed for simpler control tasks. The programmable controller had a central processing unit (CPU) and an I/O interface. The I/O interface was connected to electromechanical I/O devices on a machine or assembly line. While the I/O devices on machines retained their electromechanical character after the introduction of the PC's, the CPU of the PC replaced other electromechanical devices with its electronic hardware and programming. PC programming had its roots in the ladder diagrams which plant engineers had used prior to PC's to organize collections of electromechanical devices into control systems. It was thus different than the part programs of numerical controls which had to be developed in the applicable part program language by NC specialists.
As industry moves toward still a higher level of automation, there appears to be a number of tasks, including positioning tasks that can be advantageously performed by programmable controllers. PC's are less expensive than CNC's or other numerical controls. PC's use smaller central processing units and are adaptable to distributed processing. Programmable controllers can be programmed in the familiar ladder diagram format rather than in the part programming languages of numerical controls. And in the case of point-to-point positioning tasks, PC's can be more efficiently matched with control tasks than CNC's, which must be much more sophisticated and more expensive to provide contouring functions.